1. The initial evaluation portion of SACRED will involve obtaining TCD examinations on children with SCA between ages 3-15 years, who are followed at Hospital Infantil Robert Reid Cabral in Santo Domingo to evaluate their risk of stroke. Up to 500 patients will be enrolled. All patients, including those who are already on hydroxyurea and transfusion therapy (whether for stroke or other clinical indications), will be included to obtain a one-year cross-sectional description of TCD velocities in this patient population. Patients who are already on therapy and identified to have elevated TCD velocities will be eligible for the treatment portion of SACRED as a conditional or elevated velocity would suggest that their present therapy was not optimized.